1. Technical Field of the Invention
The present invention relates to a beam scanning optical system, and more particularly, to a structure of a beam scanning optical system incorporated in laser beam printers, facsimiles and the like for collecting a luminous flux provided with image information on a scanning medium.
2. Brief Description of Related Art
A beam scanning optical system which has heretofore been utilized in laser beam printers and facsimiles basically comprises a semiconductor laser as a light source, a deflection device such as polygon mirror and galvano mirror, and an f.theta. lens. The deflection device is utilized for deflecting a luminous flux emitted from the semiconductor laser at an equiangular velocity in a main scanning direction. With the deflection, however, a difference in a scanning speed occurs at a light collecting surface over a range from central portion to both edges in the main scanning direction and a uniform image can not be obtained. An f.theta. lens is therefore provided for correcting the difference in the scanning speed.
Since the f.theta. lens must be made by combining various concave lenses and convex lenses, lens planning is extremely complicated. Because of a number of lens surfaces to be ground, manufacturing process of the F.theta. lens requires higher accuracy and higher cost. Moreover, the f.theta. lens has a problem that various lenses of the f.theta. lens must be made of a limited material which possesses good permeability.
In U.S. Pat. No. 5,038,156, there is proposed a compact optical system wherein a spherical mirror which can be manufactured easier with higher accuracy is used in place of the f.theta. lens. Further, there are proposed optical systems wherein a toroidal lens is added to the optical system in order to rectify a tilt error of a deflection device (U.S. Pat. No. 4,984,858). On the other hand, an optical system wherein a single lens (toric lens) is provided to have F.theta. function and a tilt error correcting function is disclosed in U.S. Pat. No. 4,585,296.
However, in the optical system which is provided with the tilt error correcting function, a free space for disposing parts is reduced since a lens for the tilt error correction is added, and it greatly affects small sized printers. Being a lens, there is also a limitation with respect to its material like the f.theta. lens. If a plastic which can be manufactured easily in mass production is used, a refractive index is varied by temperature, and the refractive index of the lens is variably distributed from the circumferential portion toward the central portion since the plastic itself possesses moisture absorption characteristics. If a material such as polycarbonate which is less in moisture absorption is used, there arises a problem of double refraction in practical use. If, a glass is used, a stabilized characteristic may be obtained, however, it makes a manufacturing process difficult since the lens is not spherical to eventually raise a manufacturing cost and is not suitable for mass production.